1. Field of the Invention
The present invention relates to an electric power steering apparatus assisting a steering on the basis of an assist force of a motor.
2. Description of the Related Art
An electric power steering apparatus is generally constituted by a motor outputting an assist force capable of assisting a steering by a steering wheel, a torque sensor detecting a steering torque by the steering wheel, a computing means for computing a current command value of the motor on the basis of the steering torque, and a control means for controlling a generation of the assist force by the motor on the basis of the current command value. For example, the computing means is achieved by an electronic control unit (ECU), and the control means is achieved by a motor drive circuit, respectively. Further, the torque sensor and the ECU, the ECU and the motor drive circuit, and the motor drive circuit and the motor are electrically connected therebetween by a wire harness (a connecting means), respectively, whereby it is possible to exchange a sensor signal and various control signals and the like sent and received therebetween.
In this case, since the electric power steering apparatus constituted by hardwares such as the motor, the torque sensor, the ECU, the motor drive circuit, the wire harness and the like is generally mounted in a vehicle, there is a case that electric characteristics of the hardwares are affected by a vibration applied from an engine or a road surface, a temperature and humidity change applied from a circumferential environment, an exhaust gas, a dust or an aged deterioration, and an abnormality of the hardwares is caused. In particular, in a connector connecting the wire harnesses and a connector connecting the wire harness and a circuit board such as the ECU or the like, an electric contact failure is caused by the vibration and the aged deterioration, and there can be considered that the wire harness is damaged by a pebble or the like thrown up by a tire wheel during traveling on a gravel road or the like and the wire harness reaches a disconnection due to the damage.
For example, the abnormality of the hardware caused by the contact failure of the connector, a soldering failure of a circuit part or the like is not necessarily always generated in accordance with the condition of the vibration and the temperature and humidity change, and there is a case that the abnormality is generated and a case that the abnormality is not generated, in accordance with the condition of the time. Accordingly, in the electric power steering apparatus as mentioned above, there is employed a structure which can execute a failsafe control of setting an assist torque (an assist force) of the motor to zero at a time when an abnormality is generated, thereby preventing a secondary failure from being generated, and execute a control of rising the assist torque of the motor to a rated value in the case of determining that the state is returned from the generation of abnormality to a normal state, for example, as in “motor-driven steering apparatus” disclosed in prior art mentioned below.
Further, when rising the assist torque, the control is executed in such a manner that the torque is gradually increased to the rated value only while a driver steers, without exponentially increasing the torque, whereby a sudden change of the assist torque is prevented, and an uncomfortable feeling of the steering is not applied to the driver.
[Prior Art] JP 10-181616 A is incorporated herein by reference.
However, in accordance with the “motor-driven steering apparatus” disclosed in the prior art, since the assist torque is gradually increased without being exponentially increased at a time of rising the assist torque for returning to the normal state from the abnormal state, such a steering feeling as a steering wheel becomes suddenly light, that is, such a steering feeling as “the steering is not effective” is not applied, but the control of increasing the assist torque as mentioned above is executed limitedly at the steering time by the driver.
Accordingly, even if the hardwares are returned to the normal state, the assist torque by the motor does not rise to the rated value until the steering operation is executed by the driver. Therefore, in the case that the steering operation is not executed, the assist torque is not returned. As far as the generation of the assist torque aims to assist the steering force by the driver, it is apparently rational to limit the increase of the assist torque to the steering time on the basis of the control mentioned above. However, in the case that the abnormality is generated in the hardware under the condition that the steering operation is not executed and is immediately recovered, the steering operation is not executed by the driver in the mean time. Therefore, the assist is not executed at a time of starting the steering operation thereafter. Accordingly, since such a steering feeling as the steering wheel becomes suddenly heavy is applied to the driver in the case mentioned above, there is a problem that the deterioration of the steering feeling is caused.